1. Field of the Invention
The present invention generally relates to a method for plasma etching chromium. More specifically, the present invention provides a method for etching chromium layer through a carbon hard mask for photomask fabrication.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A mask is typically a glass or a quartz substrate that has a layer of chromium on one side. The mask may also contain a layer of silicon nitride (SiN) doped with molybdenum (Mb). The chromium layer is covered with an anti-reflective coating and a photosensitive resist. During a patterning process, the circuit design is written onto the mask by exposing portions of the resist to ultraviolet light, making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed, allowing the exposed underlying chromium to be etched. The etch process removes the chromium and anti-reflective layers from the mask at locations where the resist was removed, i.e., the exposed chromium is removed.
Another mask utilized for patterning is known as a quartz phase shift mask. The quartz phase shift mask is similar to the mask described above, except that alternating adjacent areas of quartz regions exposed through the patterned chromium layer are etched to a depth about equal to half the wavelength of light which will be utilized to transfer the circuit patterns to a substrate during fabrication. Thus, as the light is shown through the quartz phase shift mask to expose resist disposed on the substrate, the light impinging in the resist through one opening in the mask is 180 degrees out of phase relative to the light passing through the immediately adjacent opening. Therefore, light that may be scattered at the edges of the mask opening is cancelled out by the 180 degree light scattering at the edge of the adjacent opening, causing a tighter distribution of light in a predefined region of the resist. The tighter distribution of light facilitates writing of features having smaller critical dimensions. Similarly, masks used for chromeless etch lithography also utilize the phase shift of light passing through quartz portions of two masks to sequentially image the resist, thereby improving the light distribution utilized to develop the resist pattern.
In one etch process, known as dry etching, reactive ion etching, or plasma etching, plasma is used to enhance a chemical reaction and etch the patterned chromium area of the mask. Unfortunately, conventional chromium etch processes often suffer etch bias problems due to attack on the photoresist material utilized to pattern the chromium. As the photoresist is attacked during the chromium etch, the critical dimension of patterned photoresist is not accurately transferred to the chromium layer. Additionally, since etching using a photoresist mask is subject to etch bias, the use of photoresist masks for fabricating critical dimensions less than about 5 μm is extremely challenging to the fabricator as these problems result in non-uniformity of the etched features of the photomask and correspondingly diminishes the ability to produce features having small critical dimensions using the mask. As the critical dimensions of mask continue to shrink, the importance of etch uniformity dominates.
Hard masks have been recently used to provide more accurate critical dimension (CD) transfer during chromium etching for photomask fabrication. However, existing chromium etch processes have poor selectivity to carbon hard mask materials. As conventional chromium etch chemistries include oxygen, carbon hard mask materials are often etched as fast or faster than the chromium layers, resulting in unacceptable CD control, thus, making conventional chromium etch processes unsuitable for photomask fabrication.
Therefore, there is a need for a chromium etch process having high etching selectivity to carbon hard mask materials.